1. Field of the Invention
This invention relates to a thin film transistor and a photovoltaic cell using polycrystalline semiconductive films as a channel layer and a photo-activation layer, respectively, and a method of manufacturing the polycrystalline semiconductor film used for them.
2. Description of the Prior Art
Recently, a thin film transistor has been widely used as a driving element such as a liquid crystal display or a contact type image sensor. In this thin film transistor, a thin film semiconductor serves as an activation layer in which carriers move.
Since the thin film semiconductor can be formed on an insulating substrate of glass, quartz or the like and also can be formed on a relatively large substrate, unlike a conventional semiconductor formed of a single crystal, it can be applied to a device, which is rather difficult for the conventional single crystal semiconductor.
Although an amorphous semiconductor represented by an amorphous silicon film has been conventionally used as the thin film semiconductor, since carrier mobility in such amorphous semiconductive film is low because of its physical properties, its applicable field is limited.
When the thin film transistor formed of the amorphous semiconductive film can not cover the problem of the carrier mobility in the conventional case for example, it becomes necessary to add elements such that an integrated circuit (IC) is formed on the substrate as a driving element and the IC and elements formed on the substrate are connected by wire-bonding.
A polycrystalline semiconductive film has been studied for use in the place of the amorphous semiconductor. Although there are various methods of manufacturing the polycrystalline semiconductive film, a method for forming the polycrystalline semiconductive film capable of being formed over a large area at low temperature has been especially considered.
In the polycrystalline semiconductive film the carrier mobility is more than 1000 times as high as that in the amorphous semiconductive film. Thus, the aforementioned additional elements are not necessary and any element can be formed on the substrate, which reduces manufacturing costs, for example.
Meanwhile, a solar cell using the thin film polycrystalline semiconductor is highly desirable because photovoltaic conversion efficiency can be improved at low costs. According to the solar cell formed of the polycrystalline semiconductive film, it is an indispensable condition to increase a crystal grain size in the polycrystalline semiconductive film and to improve the carrier mobility in the film.
As specific methods of manufacturing the polycrystalline semiconductive film, there are proposed methods such as: chemical vapor deposition (CVD) in which the polycrystalline semiconductive film is directly formed; solid phase crystallization (SPC) in which the amorphous semiconductive film is used as an initial material and it is polycrystallized by heat treatment at approximately 600.degree. C. for several tens of hours to form the polycrystalline semiconductive film; or laser recrystallization in which the amorphous semiconductive film which is the initial material is irradiated with energy beams such as lasers to be partly melted to form the polycrystalline semiconductive film.
Generally, the grain boundaries are thought to largely influence the electrical characteristic of the polycrystalline semiconductive film. The polycrystalline semiconductive film is normally formed of many crystal grains surrounded by the grain boundaries, and the grain boundaries prevent carriers from moving in the semiconductive film. Therefore, it is important to form the polycrystalline semiconductive film so as to control generation of the grain boundaries.
In this respect, among the above several manufacturing methods, the polycrystalline semiconductive film formed by the solid phase crystallization or the laser recrystallization has usually large crystal grains, which can be of several .mu.m, so that the number of grain boundaries can be reduced in the film and preferable polycrystalline semiconductive film can be formed. For example, as disclosed in U.S. Pat. No. 5,221,365, there is proposed a method of forming a polycrystalline semiconductive film having large grains by performing heat treatment onto an amorphous semiconductive film formed on a substrate having a textured surface.
According to the above method of manufacturing the polycrystalline semiconductive film, although relatively large crystal grains can be obtained, positions of the grain boundaries can not be controlled in the polycrystalline semiconductive film. More specifically, the positions of the grain boundaries in the formed polycrystalline semiconductive film occur based on several factors, such as heat efficiency or heat conductivity in the polycrystalline film, and a surface condition or a nuclear generating position in the substrate or crystal growing speed. Therefore, the positions of the grain boundaries and the size of the crystal grain usually can not be controlled.
Consequently, if the grain boundaries exist in the polycrystalline semiconductive film serving as a channel region of the thin film transistor, the carriers are prevented from conductivity and the preferable switching characteristics are not obtained.
In addition, since the grain boundaries which do not contribute to power generation exist at random in a solar cell, power generation efficiency per unit area is lowered. Additionally, since light carriers recombine in the grain boundaries, an adverse influence is exerted upon the photovoltaic conversion characteristic.